Conventionally, various techniques for laser processing have been proposed. One of the techniques is a method of processing a workpiece using a hybrid laser beam obtained by mixing two laser beams (hereinafter referred to as a hybrid laser processing method) (for example, see Japanese Patent Laid-Open No. 2005-254328).
For example, as shown in FIG. 29, in the hybrid laser processing method, a hybrid laser beam 1 obtained by mixing a first laser beam 1a and a second laser beam 1b is applied to a workpiece 10. The first laser beam 1a is a pulse laser beam having a wavelength different from that of the second laser beam 1b. The second laser beam 1b is a CW (Continuous Wave) laser beam having a smaller application are on the workpiece 10 than the first laser beam 1a. The foci of the first laser beam 1a and the second laser beam 1b are adjusted so that the application portion of the second laser beam 1b is located in the application portion of the first laser beam 1a on the workpiece 10.
Further, in this hybrid laser processing method, a keyhole 13 is formed in the application portion of the second laser beam 1b, and the first laser beam 1a increases the growth rate and depth of the keyhole 13. Thus, even a metal material such as aluminum having high reflectivity can be welded with a sufficient penetration depth and width at high speed.
Specifically, in this hybrid laser processing method, first, only the second laser beam 1b is applied to the workpiece 10, and the application portion of the second laser beam 1b is heated by the second laser beam 1b to a temperature lower than a melting point. Then, both the first laser beam 1a and the second laser beam 1b are applied to the portion, which is heated by the first laser beam 1a and the second laser beam 1b to the melting point or higher. Then, only the second laser beam 1b is applied to the portion, which is gradually cooled by the second laser beam 1b. 
At this time, the first laser beam 1a having high intensity provides a large penetration amount at one time, and the first laser beam 1a having low intensity provides a small penetration amount at one time. The small penetration amount at one time allows the penetration amount to be accurately controlled, thereby preventing spattering. If the melted portion is gradually cooled, air bubbles generated in the portion can be removed. Further, the portion solidifies after the air bubbles are removed, thereby preventing welding defects such as porosities.
However, the conventional hybrid laser processing method has disadvantages described below.
For example, the conventional hybrid laser processing method is assumed to be used to weld the aluminum can and the sealing plate of a lithium-ion battery. In this case, if there is a gap in a part to be welded between the aluminum can and the sealing plate, the second laser beam 1b may enter through the gap into the lithium-ion battery. If both the first laser beam 1a and the second laser beam 1b enter, the first laser beam 1a and the second laser beam 1b having entered are reflected therein to damage the interior.
The conventional hybrid laser processing method is a kind of fusion welding which can reduce spattering but cannot completely eliminate spattering. Thus, spatters may enter through the gap. If spatters enter, they may cause a short circuit and ignite the lithium-ion battery.
This is because the lithium-ion battery has a higher energy density than a nickel hydrogen battery, and three times or more charging density is obtained with the same capacitance. Further, the electrode material is made of a flammable substance. Thus, the lithium-ion battery has a higher energy density and contains a flammable substance, and can easily overheat and be ignited suddenly in a short circuit.
In particular, if a large lithium-ion battery (hereinafter referred to as an on-vehicle storage battery) to be mounted in a hybrid car or an electric vehicle is ignited, the on-vehicle storage battery burns, and further the car itself burns and causes a serious accident.
Thus, in the production of an on-vehicle storage battery, when the conventional hybrid laser processing method is used, the gap in the part to be welded needs to be eliminated by fastening the periphery of the aluminum can with a machine or the like. Further, the on-vehicle storage battery is required to have no welding defect so that a liquid does not leak even if the temperature changes or the shape changes repeatedly with internal pressure fluctuations.
However, unfortunately, in a lithium-ion battery, gaps are easily created in welded portions because of the following points (1) to (4). If a gap of several tens of μm is created, a through hole is formed or a welding defect occurs, and the lithium-ion battery is rejected in a leak test.
(1) Aluminum is a metal material softer than iron. Thus, an aluminum can is warped more significantly than an iron can and easily creates a large gap. Further, a lithium-ion battery with a large size exhibits noticeable variations in the warp or mating of a part to be welded. In particular, in an on-vehicle storage battery, one side is about 10 cm. A battery with such a size exhibits large variations in warp or mating, inevitably creating a gap in a part to be welded. To fill the gap in the part to be welded, a welding width of nearly 1 mm is required. However, in the conventional hybrid laser processing method, it is difficult to completely eliminate the gap.
(2) Aluminum is a metal material having higher reflectivity than iron. Thus, spattering occurs in an aluminum can more easily than in an iron can. Thus, in the conventional hybrid laser processing method, it is difficult to prevent spattering when welding the aluminum can and a sealing plate.
(3) Aluminum is a metal material having higher thermal conductivity than iron. Thus, heat is more easily lost to the periphery of a part to be welded in an aluminum can than in an iron can. Thus, the intensity of a second laser beam needs to be increased. However, the second laser beam with increased intensity easily penetrates the part to be welded. If the part to be welded is penetrated, the interior is damaged.
(4) Aluminum is a metal material having a lower melting point than iron. Further, when aluminum is melted, the absorbance of a laser beam significantly increases. Thus, when the melting point is reached, melting rapidly progresses, thereby making it difficult to control the penetration depth of the part to be welded. The molten aluminum has high surface tension and is easily rounded. Thus, with a slight gap in the part to be welded, the end surface of the part to be welded shrinks to be rounded, thereby further enlarging the gap. As a result, perforation occurs more in the aluminum can than in an iron can. Further, since aluminum is easily oxidized, aluminum having melted and oxidized cannot be welded.
In addition to these disadvantages, in general welding, fusion welding is used when there is no gap in a part to be welded. Meanwhile, when there is a gap in the part to be welded, brazing and soldering are used. However, although spattering does not occur, the cost of brazing and soldering is high. Further, unfortunately, there is no aluminum brazing filler metal that satisfies a useful life of 15 years of an on-vehicle storage battery.